This invention relates to a method and apparatus for inspection of solder joints by X-ray fluoroscopic imaging.
With recent trends toward miniaturization and increasing density of printed circuit boards, surface mounted devices have come into wide use. Correspondingly, the solder joints have been reduced in size to become a fine configuration. Soldering inspection by use of X-ray fluoroscopic imaging is described in Japanese Patent Application Laid-open Publication No. 219632/1987 (U.S. application Ser. No. 831997); ELECTRONICS/Jul. 15, 1985, pp. 48-52; and ELECTRONIC PACKAGING & PRODUCTION/June, 1985 pp. 98-104.
Regarding the wide use of surface mounted devices, the solder joints become a fine configuration as described above, and therefore, locating a solder joint to be inspected needs be done with high accuracy. However, in the above-mentioned documents, locating the solder joint is accomplished dependent upon the mechanical accuracy of a positioning apparatus. Each solder joint location being obtained by teaching the apparatus single position points. Therefore, the mechanical accuracy is affected by the accuracy of the position points taught, the pattern accuracy of the printed circuit board to be inspected, and the overall mechanical accuracy of the inspection system itself. It has been difficult therefore to detect a fine solder joint with high accuracy.
In addition, in the aforementioned documents, no consideration has been given to a dual-sided mounted board which came into wide use of late, but rather only one-sided surface mounted boards have been taken into
The brightness of a transmitted X-ray image of a solder joint corresponds to the thickness of the solder joint. In the above-mentioned documents, a position of the solder joint cannot be decided with good accuracy for reasons described later, and the brightness only in a specific area within the solder joint is used as a reference value for comparison. Only the thickness of part of the solder joint is decided. Also, no consideration has been given to the quantity of solder nor the volume shape of fillers.
Moreover, if the X-ray is irradiated on a part for a long period of time, the part is damaged. However, in the prior art, no consideration has been given reduce the damage.